HP-41CY re-initialization ROM

[Jurgen Keller]

(11-03-2019 08:32 PM)hth Wrote:  

(11-03-2019 06:29 PM)Jurgen Keller Wrote:  W&W did for sure have such a fail-safe mechanism. Unfortunately, I didn't found any information about it. There is also a valued member of this forum who has developed such a re-initialization module in the past. However, he decided to not disclose that module for personal reasons and I fully respect his decision. Nevertheless, for personal curiosity, to get into MCODE-ing and to be able to restore my 41CY when needed, I wanted to solve the mystery.

The whole re-initialization process is not very complicated once you understand how the ENROMx instructions work in the 41CY. There is only one situation which caused me headaches:

• Assume that the 41CY "died" while bank #2 was active.
• Please note that after a ENROMx instruction is executed the next instruction is taken from the bank to which you switched to.
• It is possible to only enable odd or even pages of bank #2 but the pages of bank #1 can only be enabled together (ENROM1).
• Therefore you would end up with a situation as shown in my first picture: after the ENROM1 instruction the CPU would execute garbage which potentially crashes the calculator.
  

Not sure if that sheds some more light on it ...

After many partially working attempts the current version of my re-initialization module seems to work fine now. Hopefully, I can publish it soon.

So basically you can just use a page 4 ROM to write an ENROM1 instruction at the last address of the corrupted page which is also stuck in some secondary bank. Then call that address, bank 1 is switched in and execution falls into the next page where you have another ROM of your own, which could be just RTN (or NOP and RTN to make it safer) to pass control back again to page 4. Is that a correct understanding?

Yes, that's the trick I use to switch to bank #1 in a safe manner. I was not sure if code running across page boundaries is somehow prohibited by the HP-41 hardware. However, it seems to run fine.

BTW, I've got some insider information that W&W used a MBK ProfiSET to initialize HP-41CYs.